Litho-, bio- and chemostratigraphy across the Cenomanian/Turonian boundary (OAE 2) in the Vocontian Basin of southeastern France

The Oceanic Anoxic Event 2 (OAE 2) was one of at least two large oceanic anoxic events during the Cretaceous Period, and it is characterized in the stratigraphic record by double positive peaks in the δ¹³C curves of carbonate and organic carbon. Since the positive δ¹³C shift is believed to have resulted from massive burial of isotopically depleted organic carbon generated globally in anoxic oceanic environments, the positive δ¹³C shift tend to coincide with widespread deposition of organic-rich sediments. In the case of OAE 2, however, the organic-rich horizons do not accord exactly with the two intervals with positive δ¹³C shifts at many sites such as Pueblo, Tarfaya, Eastbourne, and Gubbio (e.g., Tsikos, H., Jenkyns, H.C., Walsworth-Bell, B., Petrizzo, M.R., Forster, A., Kolonic, S., Erba, E., Premoli Silva, I., Baas, M., Wagner, T., Sinninghe Damesté, J.S., 2004. Carbon-isotope stratigraphy recorded by the Cenomanian-Turonian Oceanic anoxic event: correlation and implications based on three key localities. Journal of the Geological Society, London 161, 711-719). The Thomel Level of the Cenomanian/Turonian (C/T) boundary in the Vocontian Basin (southeastern France) accumulated during the OAE 2, and it is marked by frequent intercalations of black shale and organic-rich marl. Detailed litho-, bio- and chemostratigraphy analyses across the C/T boundary in the Vocontian Basin indicate that the δ¹³C_carbonate profile fluctuation across the Thomel Level corresponds well with changes in lithofacies, total organic carbon (TOC), and calcium carbonate (CaCO₃) abundance. Positive δ¹³C excursions within the OAE 2 correlate with basal and middle parts of the Thomel Level that are characterized by high TOC, low CaCO₃, and predominantly contain organic-rich sediments such as black shale and dark gray marl. On the other hand, negative δ¹³C intervals within the OAE 2 correspond with lower and upper parts of the Thomel Level that consist of marly limestone and limy marl with low TOC and high CaCO₃ values. Since the fluctuation of redox conditions during Thomel Level deposition in the Vocontian Basin coincides well with the global carbon burial rate indicated by its δ¹³C profile, the Thomel Level could be an ideal candidate for the reference section of the OAE 2.